It has previously been proposed, in the U.S. application of Desvignes et al. Ser. No. 108,845, filed Dec. 31, 1979, abandoned, entitled "Copolymers of Propylene and But-1-ene and Uses Thereof", the preparation of copolymers of propylene and but-1-ene wherein the proportion by weight of units derived from but-1-ene, said proportion being denoted by the symbol (Bu), is from 10 to 40% and the structure of which is such that the proportion, denoted by the symbol "PRE", of ethyl branches which are isolated, with respect to the whole of the ethyl branches of the copolymer, is at least equal to the square of the proportion of the links derived from propylene [P] which are contained in the copolymers.
In the Examples illustrating the above-specified applications, the copolymers are prepared by the copolymerization of propylene and but-1-ene, within a liquid diluent and in the presence of a catalytic system comprising a solid compound based on titanium trichloride and one or more organo-metallic compounds of metals of groups II and III of the periodic table of elements.
In order to produce copolymers of a structure such that the proportion of ethyl branches which are isolated, with respect to the whole of the ethyl branches of the copolymers in question is at least equal to the square of the proportion of the links derived from propylene, it is necessary for the ratio between the amounts of propylene and but-1-ene contained in the polymerization medium to be maintained at a substantially constant value, throughout the polymerization operation. The above-mentioned ratio is selected on the one hand according to the but-1-ene content of the copolymer to be produced and, on the other hand, according to the catalytic systems used. The ratio can be determined experimentally. Preferably, the ratio between the amounts of propylene and but-1-ene is maintained substantially constant during the polymerization step by means of an apparatus comprising on the one hand an analyzer for measuring the ratio between the concentrations in respect of propylene and but-1-ene in the polymerization reaction vessel and, on the other hand, means for introducing propylene and but-1-ene into the reaction vessel, said means being controlled in dependence on each other and by the analyzer.
The resulting copolymers are suitable for various uses and they are particularly suitable for the production of heat-sealable films comprising a film of thermoplastic material such as isotactic polypropylene, which is coated on one face with at least one layer of said copolymer. The polyproylene film which is usually from 5 microns to 1 mm in thickness, and in most cases from 10 to 100 microns in thickness, can be produced by any known means, for example extrusion. Such heat-sealable films must be capable of being heat-sealed at fairly low temperatures, of the order of 115.degree. C. or if possible lower. It is also necessary for the heat seal to afford substantial mechanical strength as soon as it is formed, without the necessity to allow the heat-sealed film to cool completely; this latter quality, which is referred to as the hot strength of the heat seal or weld, is expressed by two temperatures which define a region within which the strength of the heat-sealed join is sufficient.
Copolymers containing more than 20% by weight of units derived from but-1-ene are particularly suitable for making heat-sealable films, because they provide the films both with fairly low heat-sealing temperatures, around 115.degree. C., and wide ranges in respect of hot strength of the heat-sealed join.